One common method of defining mass is to say that it is the quantity of matter an object possesses. For example, a small rock has a fixed, unchanging quantity of matter. If you were to take that rock to the Moon, to Mars, or to any other part of the universe, it would have the same quantity of matter—the same mass—as it has on Earth.
Mass is sometimes confused with weight. Weight is defined as the gravitational attraction on an object by some body, such as Earth or the Moon. The rock described above would have a greater weight on Earth than on the Moon because Earth exerts a greater gravitational attraction on bodies than does the Moon.
A more precise definition of mass can be obtained from Newton's second law of motion. According to that law—and assuming that the object in question is free to move horizontally without friction—if a constant force is applied to an object, that object will gain speed. For example, if you hit a ball with a hammer (the constant force), the ball goes from a zero velocity (when it is at rest) to some speed as it rolls across the ground. Mathematically, the second law can be written as F = m · a, where F is the force used to move an object, m is the mass of the object, and a is the acceleration, or increase in speed of the object.
Newton's second law says that the amount of speed gained by an object when struck by a force depends on the quantity of matter in the object. Suppose that you strike a bowling ball and a golf ball with the same force. The golf ball gains a great deal more speed than does the bowling ball because it takes a greater force to get the bowling ball moving than it does to get the golf ball moving.
This fact provides another way of defining mass. Mass is the increase in speed of an object provided by some given force. Or, one can solve the equation above for m, the mass of an object, to get m = F ÷ a. A kilogram, for example, can be defined as the mass that increases its speed at the rate of one meter per second when it is struck by a force of one newton.
In the SI system of measurement (the International System of Units), the fundamental unit of mass is the kilogram. A smaller unit, the gram, is also used widely in many measurements. In the English system, the unit of mass is the slug. A slug is equal to 14.6 kilograms.
Scientists and nonscientists alike commonly convert measurements between kilogram and pounds, not kilograms and slugs. Technically, though, a kilogram/pound conversion is not correct since kilogram is a measure of mass and pound a measure of weight. However, such measurements and such conversions almost always involve observations made on Earth's surface where there is a constant ratio between mass and weight.